These investigations are concerned with bioregulation of human disease, based on genetic studies of the mosquito Culex tritaeniorhynchus, vector of Japanese encephalitis, West Nile and Chikungunya fever viruses. This mosquito, important as a vector throughout Asia and in Africa, is used as a model species in order to discover, develop and evaluate under field conditions genetic mechanisms which may be applied in the control (by replacement or eradication) of this as well as other pest and vector species. Using the comprehensive mosquito rearing and administrative facilities available at the University of Maryland's Pakistan Medical Research Center, Lahore, together with technical staff specially trained during the preceding period of this investigation, it is proposed (a) to collect and colonize additional strains of C. tritaeniorhynchus in order to determine presence or absence of physiologic or behavioral isolating mechanisms which function as reproductive barriers (sterility), (b) to conduct basic genetic studies of new mutants of this mosquito in order to assess their value for a genetic control program, (c) to expand the study ofenzyme polymorphisms of this mosquito, with the objects of obtaining additional genetic markers to monitor a genetic control release program andof studying the genetic variability of marginal, central and isolated populations in nature, (d) to develop a system of double translocation heterozygotes for the detection and isolation of a compound chromosome strain which can be used to replace a natural mosquito population.